1. Field of the Invention
The present invention relates to a process for producing a combustor structural member that can be used (as the combustion liner and combustor basket and the like) within a combustor of a gas turbine.
2. Description of Related Art
Gas turbine combustors are used under high-temperature conditions at approximately 1,500° C. As a result of these operating conditions, such gas turbine combustors require cooling structures. For example, the Publication of Japanese Patent No. 3,831,638 (claim 9 and paragraph [0011]) discloses the use of a plate-like member having hollow passages formed therein, known as an MT fin, as the structural member for the transition piece of a combustor, wherein the gas turbine combustor is cooled by passing a cooling medium such as air or steam through the hollow passages.
As disclosed in Japanese Unexamined Patent Application, Publication No. 2002-161755 (paragraph [0007]), a plate-like member having hollow passages formed therein may be constructed from a flat plate-like member that functions as the base metal, and a plate-like member having a plurality of fins, wherein the hollow passages are formed by bonding the plurality of fins to the flat plate-like member. An example of the bonding method employed is brazing (diffusion bonding). Brazing is a technique in which a metal (the brazing filler metal) having a lower melting point than the materials to be bonded (the base metals) is used as an adhesive, and bonding is achieved by heating the base metals to the melting temperature of the brazing filler metal, thereby causing an interdiffusion effect between the brazing filler metal and the base metals. If the heating temperature during brazing is too high, then oxidation and coarsening of the crystal grains of the base metals may occur, resulting in a deterioration in the strength of the base metals. Accordingly, brazing filler metals containing one or more added melting point lowering elements (such as B, Si or P) are widely used to reduce the heating temperature.
Conventionally, the aforementioned structural members for gas turbine combustors are produced by brazing two plate-like members together, and subsequently performing molding at normal temperatures using a pressing machine. During the brazing, the melting point lowering element(s) contained within the brazing filler metal diffuse out from the contact surfaces between the brazing filler metal and the plate-like members. However, in those cases where this diffusion is insufficient, the melting point lowering element(s) tend to become concentrated on the plate-like members near the above contact surfaces, resulting in localized brittleness. If a plate-like assembly formed from such plate-like members is subjected to press molding, then cracks may develop within the interior of the plate-like assembly, and these cracks tend to cause problems of detachment at the bonded surfaces of the plate-like assembly. Consequently, the internal quality of combustor structural members formed by molding of plate-like assemblies has needed to be checked by nondestructive testing (such as ultrasonic testing). If cracks are detected in a combustor structural member, then the combustor structural member must be either repaired by welding or discarded.
If the plate-like assembly having hollow passages formed therein is molded using a hot forging press machine, then the problem of cracks occurring within the interior of the plate-like assembly can be avoided. However, because this technique requires heating at a high temperature, the brazing filler metal and the plate-like members tend to melt, which makes it difficult to maintain the positional precision of the hollow passages, and tends to cause the plate-like members to suffer from coarsening of the crystal grains, resulting in a deterioration in the strength of the plate-like members. Hot press molding is a technique in which the entire material that is to be molded, such as a non-iron metal, stainless steel, alloy steel or carbon steel, is heated in a heating furnace or the like, and is subsequently subjected to compression molding using a die. In contrast, cold press molding is a technique in which the material to be molded is subjected to compression molding using a die at ambient temperature.